Wash the potatoes thoroughly and boil them in salted water for about 25 minutes. Drain and cool and peel. Peel asparagus and cut off woody ends. Bring water with sugar and lemon juice to the boil and simmer asparagus for about 20 minutes.
Drain and let drain. In the meantime crumble the toast. Cut salmon into small cubes. Melt butter. Mix crumbs, salmon and butter. Season with pepper. Cut potatoes into slices.
Grease the casserole dish and lay potato slices on the sides like roof tiles. Place asparagus in the middle and sprinkle with smoked salmon crumbs. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for approx. 20 minutes.
Garnish as desired with chervil.